The purpose of this study is to examine if the correct use of antibiotics, as well as the application of one-day-treatment, in the therapy of periodontal diseases. The population of treated patients (184) are divided in 4 groups; Group 1: Metronidazole + mechanical treatment in one day, Group 2: Placebo + mechanical treatment in one day Group 3: Metronidazole + mechanical treatment on day 1 and 21 Group 4: Placebo + mechanical treatment on day 1 and 21 End points, BOP, Plaque, PPD, CAL, RAL, Microbiology will be recorded on all sites at 3 months, 1 -, 3 -, and 5 years post-treatment